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Predicting the Performance of Mini Implant-Retained Prostheses Using Finite Element Analysis
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OVERVIEW R&D Corner: When using mini implants to support a denture, the quantity of implants used can vary; however, a sufficient quantity of implants must be placed to adequately distribute the forces generated during chewing to prevent fracture of the implants. Glidewell Laboratories Director of Implant R&D and Digital Manufacturing Grant Bullis and Vaheh Golestanian, manufacturing engineer in the lab's Implant R&D and Digital Manufacturing department, simulate how finite element analysis can be used to approximately predict how implant-supported prostheses will perform in the mouth.
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Grant Bullis
Manager, Research & Development, Glidewell Laboratories
Newport Beach, CA
800-839-9755 Ext.1948
inclusivemagazine@glidewelldental.com |
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| Grant Bullis, Glidewell Laboratories Research & Development Department Manager, began his career in the dental industry at Steri-Oss in 1997. After Nobel Biocare acquired Steri-Oss, Grant worked in the R&D Department, where he was responsible for the development of implants, prosthetics, surgical tools and packaging. Grant, who joined Glidewell Laboratories in 2007, now manages CAD/CAM and implant product development at the lab. He directs manufacturing for more than 150 implant laboratory and prosthetic components. Grant has a degree in mechanical CAD/CAM from Irvine Valley College in Orange County, Calif., and an MBA from Keller Graduate School of Management. |
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Vaheh Golestanian
Manufacturing Engineer, Glidewell Laboratories
Newport Beach, CA
inclusivemagazine@glidewelldental.com |
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| Vaheh Golestanian received a master's degree in biomedical engineering at Iran University of Science and Technology in Tehran. In 2008, he joined Glidewell Laboratories' Implant R&D and Digital Manufacturing department as a manufacturing engineer. Vaheh has eight years' experience as a mechanical engineer focused on finite element analysis and CNC programming. A member of the Society of Manufacturing Engineers, he recently co-authored a technical paper on using finite element methods to design zirconia abutments. |
PAGE 1 OF 4 Discussion
Patients with resorbed residual ridges may not have adequate bone volume for standard implants. If site augmentation isn't an acceptable option for the patient, then small-diameter "mini" implants can be used to retain a prosthesis when there is sufficient bone volume and density for primary stability.
Mini implants can provide anchorage points for orthodontic treatment and retention for removable prostheses, such as dentures. They are indicated for several situations where conventional implants are not suitable1:
- For patients with inadequate bone width
- For older or medically compromised patients (flapless insertion of a mini implant preserves continuous blood flow to the area)
- For financially challenged patients who cannot afford conventional implant treatment
- For patients who are unwilling to undergo extensive bone augmentation
- For patients who are unwilling to wait the several months of healing frequently associated with conventional implant treatment
When mini implants are used to retain a denture, a sufficient quantity of implants must be placed to adequately distribute loads generated during mastication. Using multiple mini implants to retain removable prostheses reduces the forces experienced by individual implants. If too few implants are used, cyclic occlusal loading may fatigue the small-diameter implant neck to the point of fracture. 2 The quantity of mini implants used can vary, but four implants in the anterior mandible is a common configuration.
Patients with implant-retained mandibular overdentures opposing complete maxillary dentures can generate significantly higher maximum bite forces than those with conventional complete dentures.3
PAGE 2 OF 4 Care must be taken with prosthesis design and implant placement to minimize off-axis loading. The prosthesis may be relieved in centric occlusion to reduce cyclic loading and occlusal force impact.2
One method available to analyze occlusal force transfer from the prosthesis to the implants and the supporting bone is finite element analysis (FEA). The history of FEA dates back to 1943, when Richard Courant used triangular mesh elements to study torsion of a cylinder.4 There are several FEA software programs currently available. They are used extensively by the automotive and aerospace industries to simulate, visualize and optimize structures for strength and stiffness.
Human tissue and bone are challenging to model accurately. Bone is not a homogeneous material, and its quality and quantity can vary considerably between individuals. When applying FEA to implant biomechanics, some simplifications and assumptions are necessary in order to build a model that can be solved. These assumptions can have a significant effect on the accuracy of the results. They include5:
- The detailed geometry of the bone and the implant to be modeled
- Material properties
- Boundary conditions
- The interface between the bone and the implant
FEA gives engineers and product designers a powerful tool to evaluate the efficacy of their designs under simulated use conditions. It allows them to apply boundary conditions and forces to their designs and evaluate their expected performance. At Glidewell Laboratories, we use this technology to assess product designs before, and in conjunction with, physical prototypes. 6
PAGE 3 OF 4
Simulation Techniques
For our simulation, we used a stereolithographic (STL) model of a mandible created from an optical scan. The STL model was imported into a CAD program. Four 2 mm mini implants were placed anterior to the mental foramina. Cylindrical features with diameters and depths corresponding to the surgical site preparation were subtracted from the implant sites.
The model was printed using a rapid prototyping machine. The printed model had four holes at the locations of the implants. Mini implants were placed in these locations (Fig. 1). Soft tissue was then added over the implant sites (Fig. 2).
A denture was then fabricated to fit the model (Fig. 3). O-ring housings typical of those used to retain dentures were incorporated into the denture at each implant site.
Next, an STL model of the denture was created from an optical scan. The model was then imported into a CAD program. In the CAD program, all the parts (jaw, implants, O-rings, implant housings and denture) were assembled (Fig. 4).
Boundary Conditions and Meshing
The mandible was restrained at the temporomandibular joint. A force of 200 N was applied approximately at the locations shown by the arrows (Fig. 5). The assembly was meshed and refined. A default element size of 2.5 mm was defined.
Analysis and Results
After establishing the boundary conditions and meshing on the assembly, FEA was conducted on the assembly. The analysis took approximately 1.5 hours to complete due to the complex shape of the model and the small size of the elements. The results of the FEA indicated that bite forces are distributed relatively evenly across the implants, except when force is applied at the middle of the denture, and that stresses are higher in the outer implants compared to the middle implants (Fig. 6).
PAGE 4 OF 4
Conclusion
The results of the simulation indicate a safety factor of 3x relative to the strength of the implants when a 200 N force is applied across a mandibular overdenture retained by four mini implants in the symphysis region. The number of implants, bone quality and anterior-posterior spread of the implants are significant factors in how effectively occlusal loads are distributed. It must also be considered that occlusal loads that are significantly divergent to the axes of the implants introduce stress gradients at the implant sites that are not apparent in this simulation.
The forces that act on removable implant-retained prostheses have complex spatial and temporal distributions, which currently are not practical to define and simulate. The prosthesis incorporates compliant attachments to the implants that undergo semi-restrained axial loading. Defining the bone-biomaterial interface also presents significant challenges. The displacements at the interface are small. In addition, bone is not a homogeneous material and its structural characteristics can vary greatly between patients. To simplify the model enough to perform an analysis, assumptions have to be made that affect the accuracy of the results.
More research on the composition and simulation of the interaction of anterior excursive forces on implant-retained overdenture is necessary to better approximate actual-use conditions. Finite element analysis is an approximate method of predicting how implant and prosthetic designs will perform. Testing is ultimately necessary to confirm that the results of the analysis are predictively valid.
References
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